Send installed language data to the plugin and theme update-check endpoints.

Introduces wp_get_installed_language_data() and wp_get_pomo_file_data().

see #18200.

Built from https://develop.svn.wordpress.org/trunk@25520


git-svn-id: http://core.svn.wordpress.org/trunk@25440 1a063a9b-81f0-0310-95a4-ce76da25c4cd